1. Field of the Invention
The present invention relates to a technology for measuring a capacitance value of a touch screen, and more particularly, to a circuit and a method for measuring a capacitance value of a touch screen, which can linearly change a reference voltage of a comparator used for detecting a capacitance value, to improve a resolution, and can use subranging to shorten a time for measuring a capacitance value.
2. Description of the Related Art
Recently, touch screens are frequently used as a user interface device. Touch screens are divided into a resistive film type, a capacitance type, an infrared type and an ultrasonic type, depending upon a panel type. A touch screen of the capacitance type has advantages in terms of high transmittance, durability and price, and thus, is widely used in a mobile phone, a portable terminal such as a personal digital assistant (PDA), a monitor, and a variety of electric appliances. Hereafter, the touch screen of the capacitance type will be described.
A user can issue a desired command by touching an optional point on a touch screen display device using a stylus pen or a finger. To this end, the touch screen display device includes, in addition to a plurality of pixels for displaying an image, a plurality of touch sensing elements for sensing the point which is touched by the user.
A gate signal and a data signal are applied to each pixel, and each touch sensing element senses a touch made by the user and outputs a resultant sensing signal. To this end, the touch screen display device a gate driving unit and a data driving unit for applying the gate signal and the data signal, and a touch signal recognition circuit for recognizing a touching operation based on a change in the capacitance of a capacitor by the touch.
If the touch screen is touched by a body portion such as a finger or a stylus pen, capacitance of a capacitor for sensing a touch (hereinafter, referred to as a “target capacitor”) changes depending upon an intensity of the touch. For example, as an intensity of a touch is great, capacitance increases. At this time, because a capacitance change is fine to several pF to several tens pF and parasitic capacitance on a touch screen pad is larger than several tens pF, a capacitance variation becomes relatively small. Thus, in order to obtain a high resolution, a clock signal with a high frequency is required.
FIG. 1 is a circuit diagram illustrating a conventional circuit for measuring a capacitance value of a touch screen.
Referring to FIG. 1, the conventional circuit for measuring a capacitance value of a touch screen includes a target capacitor unit 110, a target voltage control unit 120, a reference voltage supply unit 130, a comparator 140, and a controller 150.
First, a target capacitor initialization switch SW121 is turned on by a first switching control signal CS1 outputted at a high level from the controller 150. As a consequence, charges charged in a target capacitor Ctarget and a parasitic capacitor Cpad arranged at a touched point are completely discharged, and thus, the target capacitor Ctarget and the parasitic capacitor Cpad are initialized.
The target capacitor Ctarget is one of a plurality of touch lines which are arranged in one direction, for example, a vertical direction, on a touch screen panel, or one of a plurality of target capacitors which are arranged on the one touch line. Further, the parasitic capacitor Cpad is a capacitor present in a pad PAD to which one terminal of the target capacitor Ctarget is connected.
Thereafter, the target capacitor initialization switch SW121 is turned off by the first switching control signal CS1 outputted at a low level from the controller 150, and a target capacitor charging switch SW122 is turned on by a second switching control signal CS2 outputted at a high level from the controller 150. Accordingly, as the target capacitor Ctarget and the parasitic capacitor Cpad are charged by a target current source 121, a voltage of the pad PAD as a common connection point of the target capacitor Ctarget and the parasitic capacitor Cpad increases linearly with the lapse of time.
The comparator 140 compares the voltage of the pad PAD inputted through the non-inverting input terminal thereof and a reference voltage of a fixed level inputted through the inverting input terminal thereof, and outputs a transition signal at a moment when the voltage of the pad PAD becomes higher than the reference voltage Vref. As a user touches or does not touch the target capacitor Ctarget, capacitance of the target capacitor Ctarget changes, and due to this fact, the voltage inputted to the non-inverting input terminal of the comparator 140 through the pad PAD changes. Therefore, depending upon a capacitance value obtained as user touches or does not touch the target capacitor Ctarget a time, at which the comparator 140 outputs the transition signal, varies.
The controller 150 receives the transition signal outputted from the comparator 140 in synchronization with a clock signal CLK, calculates a capacitance value based on a time elapsed from the time at which the target capacitor Ctarget is initialized to the time at which the transition signal is received, and outputs a corresponding digital code. The controller 150 may include a digital counter or a TDC (time-to-digital converter).
FIG. 2 is a graph showing voltage changes in the pad PAD depending upon whether or not the target capacitor Ctarget is touched. In FIG. 2, G21 is a graph showing a voltage change in the pad PAD when the target capacitor Ctarget is not touched, and G22 is a graph showing a voltage change in the pad PAD when the target capacitor Ctarget is touched.
When the target capacitor Ctarget is touched by a user, capacitance changes. By this fact, as can be seen from the graph G22, the voltage slope of the pad PAD decreases, and a time for reaching the reference voltage Vref is lengthened. This is expressed as in the following Mathematic Equations 1.
                                                        t              1                        =                                                                                C                    target                                    +                                      C                    pad                                                                    I                  target                                            ⁢                              V                ref                                              ⁢                                          ⁢                                    t              2                        =                                                                                C                    target                                    +                                      C                    pad                                    +                                      Δ                    ⁢                                                                                  ⁢                    C                                                                    I                  target                                            ⁢                              V                ref                                              ⁢                                          ⁢                                    Δ              ⁢                                                          ⁢              t                        =                                                            t                  2                                -                                  t                  1                                            =                                                                                                                  Δ                        ⁢                                                                                                  ⁢                        C                                                                    I                        target                                                              ⁢                                          V                      ref                                                        ⁢                                                                          ∴                                      Δ                    ⁢                                                                                  ⁢                    t                                                  =                                                                            V                      ref                                                              I                      target                                                        ⁢                  Δ                  ⁢                                                                          ⁢                  C                                                                    ,                                  ⁢                                            Δ              ⁢                                                          ⁢              t                                      t              1                                =                                    Δ              ⁢                                                          ⁢              C                                                      C                target                            +                              C                pad                                                                        [                  Mathematical          ⁢                                          ⁢          Equations          ⁢                                          ⁢          1                ]            
Here, ΔC is a capacitance variation in the target capacitor Ctarget by the touch, and Δt is a time t2−t1 additionally required to reach the reference voltage Vref due to the capacitance variation resulting from the touch.
A maximum conversion time tconversion—max required to measure the capacitance variation of the target capacitor Ctarget is expressed as in the following Mathematical Equation 2.
                    ⁢          [              Mathematical        ⁢                                  ⁢        Equations        ⁢                                  ⁢        2            ]                  t      conversion_max        =          (                                                  V              ref                                      I              target                                ⁢                      (                                          C                target_min                            +                              C                pad                            +                              Δ                ⁢                                                                  ⁢                                  C                  max                                                      )                          +                  t          reset                +                  t          digital                    )      
Here, treset is a time required to initialize the target capacitor Ctarget and the parasitic capacitor Cpad, tdigital is a time elapsed from a time at which a voltage inputted to the non-inverting input terminal of the comparator 140 is increased over the reference voltage Vref to a time at which the digital code is outputted from the controller 150.
In the case where the controller 150 includes a digital counter, a frequency fCLK of the clock signal CLK required to attain a desired n-bit resolution is expressed as in the following Mathematical Equation 3.
                                          f            clk                    =                                                    2                n                            -              1                                      Δ              ⁢                                                          ⁢                              t                max                                                    ,                              Δ            ⁢                                                  ⁢                          t              max                                =                                                    V                ref                                            I                target                                      ⁢            Δ            ⁢                                                  ⁢                          C              max                                                          [                  Mathematical          ⁢                                          ⁢          Equations          ⁢                                          ⁢          3                ]            
While Δtmax should be increased to decrease the frequency fCLK of the clock signal CLK, it can be seen that Δtmax has a direct trade-off relationship with the maximum conversion time tconversion—max. Δtmax means a maximum time additionally required for the output voltage of the pad PAD to reach the reference voltage Vref as a capacitance variation becomes a maximum by the touch.
Accordingly, in the conventional circuit for measuring a capacitance value of a touch screen, difficulties exist in decreasing a frequency of a clock signal which is required to improve a resolution or attain a predetermined resolution. Due to this fact, the stable operation of the circuit cannot be ensured, and it is difficult to simplify the configuration of the circuit and reduce power consumption.